Transposable elements are mobile units of DNA capable of movement to new chromosomal positions. Several categories of transposable elements, with different structures and abilities, have been found in Drosophila, where they constitute at least five per cent of the DNA, and play important roles in the structure, function and evolution of the genome. We propose to extend our knowledge of the mammalian transposable elements, about which relatively little is now known. We have recently developed a search procedure, based on phylogenetic DNA differences, that has enabled us to isolate several new families of human and mouse putative transposable elements. These will first be analyzed by the standard procedures of restriction mapping, DNA sequencing and Southern blot hybridization. In studying the human elements we will be particularly interested in finding new categories of mammalian elements and in beginning to catalogue the numbers and types of elements present. Evidence for mobility will be sought by using Southern blot procedures to look for individual differences in element positions. It is hoped that this study will lead to a better understanding of the function of transposable elements in the human genome, and to a practical and rapid method for identifying medically useful restriction fragment length polymorphisms. Transposable elements from the deer mouse will be microinjected into house mouse embryos, where they are not normally found, to determine if they are still actively mobile. If appropriate they will be further analyzed by microinjecting elements altered by in vitro mutagenesis, to identify functional sequences. It is hoped that these elements will prove to be useful mutagens, that tag genes of interest, and gene transfer vectors for use in mammalian systems. We will directly study the coding functions of transposable elements by subcloning open reading frames into bacterial plasmid expression vectors that will allow the production of the corresponding protein. We have now used this approach to make protein from the open reading frame we recently found in a human Kpn I element, and this protein has been used to raise polyclonal antibodies, for study of the cellular counterpart protein. Open reading frames from Drosophila foldback or HB elements, or from corresponding cDNA clones, will be used to make protein which will be tested for specific DNA binding and enzymatic activity.